1. Field Of The Invention
This invention relates to improved methods and compositions for investment casting technology. The invention preferably involves improved phosphate modified aluminum salt binder compositions and their use in a process for forming quick setting refractory laminates. This process is particularly suited to the manufacture of expendable laminar ceramic shell molds for the investment casting of metal parts.
2. Description Of The Prior Art
Investment casting by the lost wax or cire perdu process is an old art that can be traced to ancient Egypt and China as early as 400 B.C. The process as practiced today, however, is a relatively new technology dating to the 1930's and represents a rapidly growing business and science. This technology simplifies the production of complex metal shapes by casting molten metal into expendable laminar ceramic shell molds formed around disposable wax patterns duplicating the desired metal shapes. The term "Investment Casting", derives from the Latin "investire" meaning to clothe, cover or surround and refers to the ceramic shell mold that is formed around the disposable wax pattern. "Precision Investment Casting", or PIC, is often the term preferred in the art to refer to the technology.
The conventional PIC process consists of six major steps:
(1) Pattern preparation--disposable positive patterns of the desired metal casting are made from thermoplastic materials (usually wax) that will melt, vaporize or burn completely so as not to leave contaminating residues in the de-waxed ceramic shell mold. The positive pattern is prepared by injecting the material into a negative, segmented, metal die or "tool" designed to produce patterns of the shape, dimension and surface finish required for the metal casting. Single or multiple patterns are then assembled by carefully fusing to a disposable wax "sprue system" that serves to properly feed molten metal to fill the shell mold;
(2) Investiture--the laminar shell mold construction consists of:
(a) dipping the wax pattern assembly in a slurry consisting of fine particulate refractory grain in an aqueous solution of alkali stabilized colloidal silica binder to define a coating of slurry on the pattern surface; and PA1 (b) contacting the slurry coating with coarse dry particulate refractory grain or "stucco" to define a stucco coating, followed by PA1 (c) carefully air drying to define an air dried insoluble gel bonded coating. These process steps are then repeated to build by successive coats a laminar shell mold of the desired thickness. PA1 (1) This invention teaches the use of a binder rather than a salt. It also teaches a preferred method for converting an aluminum salt into a binder by the addition of phosphoric acid; this conversion method insures solubility within useful ranges and is both convenient and economical. PA1 (2) Any soluble acidic aluminum salt can be used as the starting salt and need not contain hydroxyl groups. PA1 (3) Two or more acid anions are present in the "converted" salt. PA1 (4) Concentrations and ratios of phosphate to the soluble aluminum salt are defined and related to binder utility in both green and fired strength of the resultant shell molds. PA1 (5) This invention makes possible the use of two interacting binder slurries so that each dip in the shell making process defines a gel bonded coating layer that is productive in binding shell thickness. PA1 (6) Use of dry stucco between slurry coats with the double slurry process improves shell mold uniformity and eliminates excess water which must be removed and is detrimental to shell mold construction.
(3) Dewaxing--The disposable wax pattern assembly is removed from the "green" air dried shell by steam autoclaving, or by plunging into a flash de-waxing furnace heated to 1000.degree.-1900.degree. F. or other method for rapid heating to liquify the wax so that excessive pressure build-up does not crack the shell mold.
(4) Furnacing--The de-waxed shell mold is heated at a temperature of 1600.degree.-2000.degree. F. to remove volatile residues and form stable ceramic bonds by sintering. The furnaced shell mold temperature and heating cycles are carefully controlled to meet metal pouring requirements.
(5) Pouring--The pre-heated shell mold is recovered from the furnace and positioned for receiving the molten metal pour. The pour may be made in the air or vacuum chamber. The metal charge may be melted by gas, indirect arc, or induction heating. The castings may be poured statically or centrifugally and from a ladle or direct melting crucible.
(6) Casting recovery--The cast shell molds are broken apart and the metal castings are separated from the ceramic shell material. The casings are separated from the sprue system by sawing or cutting with abrasive disks. Castings are then cleaned by tumbling, shot or grit blasting.